Replace the legacy Fabric Crashlytics SDK with the new Firebase Crashlytics SDK

[uhrohraggy]

Fabric and Crashlytics dependencies are deprecated as of October 2020, and just now Nov 15 , the project will fail with error:

groovy.lang.MissingPropertyException: No such property: scope for class: com.android.build.gradle.internal.variant.ApplicationVariantData

as described here: https://stackoverflow.com/questions/62198361/no-such-property-scope-for-class-com-android-build-gradle-internal-variant-app

Fix is to replace with the new FirebaseCrashlytics per https://firebase.google.com/docs/crashlytics/upgrade-sdk?platform=android#java

I've started this replacement, should have a PR up tonight.